This invention relates to the imaging method utilizing the magnetic resonance (MR), and more particularly to an MR imaging method for scanning of a cross-section of a living subject, adapted to a method of visualizing of blood vessels.
Techniques of angiograhy by utilizing the magnetic resonance are disclosed in detail in IEEE, TRANSACTIONS ON MEDICAL IMAGING, VOL. MI-5, No. 3 (1986) pp. 140-151.
Basically, there are two methods; subtraction method and cancellation method. In both methods, pulses called flow encode pulses are applies to a moving fluid in a direction in coincidence with the flow component direction of the fluid.
The flow encode pulses are added not to give affects to the gradient magnetic field which is usually used for taking a tomography picture. The flow encode pulses are two gradient magnetic fields of the same magnitude and the positive and the negative polarities. After the gradient magnetic field of the positive polarity is applied, the gradient magnetic field of the negaive polarity is applied after the lapse of a predetermined time. When the flow encode pulses are applied to a stationary subject, the phase of a spin is first varied by the positive gradient magnetic field and then is returned to the original position by the negative gradient magnetic field. Thereby, the phases are aligned as the result and when the phases are aligned the signals are detected. Where as when the flow encode pulses are applied to a moving fluid such as blood in the same direction as the flow direction component, the spins do not receive the action of the same magnitude by the negative gradient magnetic field as the spin changes which they received by the positive gradient magnetic field, since the spins which are given phase shifts by the positive gradient magnetic field change their position by the flow. Therefore, the phase shifts are not completely cancelled and there remain phase shifts of the magnitude corresponding to the difference of the positive and the negative gradient fields. Further, since the blood flow is a laminar flow, the changes of the phase shifts are different at every position in a blood vessel and the phase are dispersed and not aligned. When the phases are not aligned, the resonance signal becomes weak. Thus, the spins of a fluid cause phase changes after the application of the positive and the negative gradient magnetic fields. Discrimination of the fluid from the stationary portions can be done by detecting the changes of the resonance signal by those phase changes. Thereby, blood vessels can be imaged.
Based on this principle, in the subtraction method, subtraction is done between the reconstruction images of the flow-sensitive sequence including the flow encode pulse and the flow insensitive sequence not including the flow encode pulse. In the blood vessels, blood flows in laminar flow as shown in FIG. 4B. Therefore, when a tomography is taken in the flow-sensitive sequence, different phase changes arise in various positions in the blood vessel and the projection data integrating those results produces no signal from the blood vessel by the mutual cancellation.
In the stationary portion, since the changed phase is returned to the original position, the resonant signal is detected when the phases are aligned.
On the other hand, in the flow-insensitive sequence, similar resonant signals are obtained from the flowing portion and from the stationary portion.
When subtraction is made between the two sequences, the stationary portions disappear and the blood vessel portions which are the difference of the two sequences appear.
The cancellation method is a method of obtaining a blood vessel image by one measurement. Upon exciting the spin, an RF pulse corresponding to a 360.degree. pulse having an intensity of four times as strong as a 90.degree. RF pulse is applied, and a flow encocde pulse is applied simultaneously. In the stationary portion, the spins are returned to the original positions and no signal is generated. In the moving portion, however, the phases are changed by the flow encode pulse and hence a signal is generated. Thus, when the observation signals are converted into a picture, only the blood vessels are obtained.
Those related arts, however, are insufficient with respect to the treatment of the laminar flow. Namely, when a laminar flow is measured as a projection data, good signals are not necessarily derived from blood vessels. First, in the subtraction method, it is assumed that the phases in the blood vessels do not change in the flow-insensitive sequence not including the flow encode pulse. This assumption holds only for a flow of a constant velocity. In the actual blood flow in arteries, however, the flow velocity changes rapidly in synchronism with the contraction of the heart and hence includes acceleration components which vary the velocity with the time. Therefore, being different from the case of a flow of a constant velocity, there arise phase changes. Further, even if the magnitude of the velocity is constant, when the blood vessel is curved, the blood flow therein will include the acceleration component and also produce the phase change. Therefore, even when an image is taken in the flow-insensitive sequence, the resonance signal in the blood vessel is weakened by the phase change.
Therefore, it has been difficult to extract arteries of complicated shape in the head such as medium and large brain arteries. Further, the subtraction method requires two picture-taking processes, flow-insensitive and flow-sensitive, and hence requires a long time for measurement. Especially, a long measurement time is required for the three dimensional imaging method which requires many sheets of depthwise information.
On the other hand, in the cancellation method, the flow sensitive sequence is basically used. Even when signals are generated only from moving spins, signals from a laminar flow cancel each other because of the phase differences in the laminar flow. As the result, the resonant signal becomes very weak and it has been basically difficult to extract the arteries in the brain.